Period and cohort effects: consequences on spirometric lung function in Norway during the 20th century.

Abstract

Several factors can influence measured lung function over time. The aim of this study was to investigate period and cohort effects on spirometric measures in a large general population sample in Norway during the 20th century, using Global Lung Function Initiative (GLI-2012) equations as a reference. 36 466 subjects (born 1894-1969) from four cross-sectional surveys conducted between 1965 and 1999 were included, with harmonised data on smoking habits, respiratory symptoms, lung diseases, education and spirometry. Changes in forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) z-scores in healthy subjects across surveys were explored to investigate period effects. Linear mixed-effects models of FEV1 and FVC z-scores on birth cohort, with survey as random effect, were used to investigate cohort effects, both in subjects of the total population and in healthy ones. Relatively higher FEV1 and FVC z-scores in healthy subjects were found in the first survey (1965-1970) compared to the more recent ones (1988-1999), suggesting period effects. FEV1 and FVC z-scores increased significantly with birth cohort from 1894 to 1935, after adjustment for covariates. A more stable trend of FEV1 and FVC z-scores with birth cohort was evidenced for subjects born more recently (1945-1969). An increase of lung function with year of birth was observed in Norwegian subjects during the first half of the 20th century. The impact of period effects on lung function decreased from 1965 to 1999.